The present invention relates to a method for modifying surface properties of a shaped article of a silicone or, in particular, to a method for imparting durable affinity to water to the surface of a silicone rubber article.
Shaped articles of silicones, e.g. silicone resins and silicone rubbers, are widely used in various fields of industry in general owing to their excellent properties such as anti-weathering resistance, heat resistance, workability, mechanical properties, electrical properties and the like. Recently, silicone products have become a promising material and used in the medical field by virtue of their physiological inertness to the human body.
A problem in the use of silicone products is that the surface of a silicone article is generally water-repellent inherently so that the use of silicone products is necessarily limited when the surface of the article is desired to be wetted with water as is frequently the case in the medical field or others.
Such a problem of poor affinity to water and a problem of accumulation of static electricity on the surface are not limited to the silicone products but are generally encountered in most of synthetic polymer products. Several attempts have been made hitherto to modify the surface properties of shaped articles of plastic materials to be imparted with increased affinity to water. One of the approaches is to coat the surface of the article with an anti-static agent. This method is effective when the anti-static effect is desired to be instantly exhibited but is defective in the poor durability of the effect and the stickiness on the surface with eventual blocking of the coated articles in contact with each other. Alternatively, a method is proposed in which the shaped articles are fabricated with the resin or rubber composition admixed with an anti-static agent. The effect of this method is considerably durable and the surface resistivity of the shaped articles can be decreased to some extent but this method is impractical due to the insufficient affinity to water of the surface and low anti-static effect. An attempt to increase the amount of the anti-static agent in the resin or rubber composition is unsuccessful because of the appearance of stickiness on the surface leading to eventual blocking of the articles and the problem of bleeding or blooming of the anti-static agent per se on the surface in addition to the lowering of the heat resistance and workability as well as the surface coloring and increased susceptibility to stain.
It has been recently disclosed that the affinity to water of the surface of a plastic shaped article can be increased when the surface is exposed to an atmosphere of low temperature plasma leading to the decrease in the static electricity on the surface. This method of plasma treatment has not yet been established because of the relatively low effectiveness of the method. Moreover, when the surface of a silicone product is treated with low temperature plasma, affinity to water is indeed increased to some extent but the durability of the effect is low so that the once obtained affinity to water is rapidly lost in the lapse of days as has been recognized in the experiments by the inventors.